The 19-year-old - a £1.2million arrival from Hamilton last summer - overcame a difficult start to his career in England, when he suffered from homesickness and a number of niggling injuries before Christmas.
"Everything about last season was a highlight, to be honest. It was just a great season for me," McCarthy told the Wigan Evening Post.
"To be playing in the Premier League against the top players was a dream come true - something I wanted since I was a kid.
"I just wanted to get as many games under my belt as I could, because I had played over a hundred times for Hamilton in the SPL.
"It wasn't to be for the first six months, but I finally got my chance and I was delighted to stay in there.
"That's over and done with now, though. It's all about this year and I know it's going to be even harder to stay in the team."